Wireless radio frequency (RF) communications may employ a time division sharing or multiplexing of a common antenna by RF transmitting units and RF receiving units. For purposes of this description, the term “antenna,” standing alone, means at least major radiating element(s) of what may be a larger structure. Such time division sharing may be employed by, for example, the RF receiving and RF transmitting units forming the transceiver systems within a base station of a wireless communication system, e.g., a cellular telephone system. Because the sharing of a common antenna is between signals traveling in opposite sending and receiving directions, this may be referred to as Time Division Duplex (TDD) sharing.
To achieve a desirable coverage area, antenna are often positioned a considerable height above ground. The positioning may be accomplished by, for example, mounting the antenna on a tower. In such a mounting arrangement, the RF receiving unit may not be positioned proximal to the antenna feed. Instead, it may be located at a distance away that is at least equal to the height of the antenna. However, as known in the RF communication arts, propagating an un-amplified reception signal from an antenna feed to a receiver located a significant distance away may results in a signal, when arriving at the receiver, having substantially lower signal-to-noise ratio (SNR) that exhibited at the feed. To avoid such degradation, an RF amplifier may be arranged between the feed port of the antenna and the input of the RF receiving unit, configured to amplify the RF signal received from the antenna feed port into a higher power signal, thereby reducing the degradation in signal-to-noise ratio (SNR) caused by additive noise. Typically, for reasons well known to persons skilled in the relevant arts, the RF amplifier that amplifies the RF signal received from the antenna feed port for transmission to the RF receiving unit is a low noise amplifier (LNA).
The receiving RF LNA, however, often imposes a requirement that signal energy from the RF transmitter does not couple onto input of the RF LNA during operation of the RF receiver unit. Such energy would be amplified by the RF amplifier and then input to the RF receiver unit.
One known arrangement for reducing RF transmitter energy coupling into the receiving RF LNA input is a switching circuit that alternately connects the output of the transmitting unit and the input of the RF LNA amplifier to the feed terminal of the antenna. For brevity and consistency, this prior art switching arrangement will be arbitrarily labeled as a “switch-based TDD RF LNA isolator.”
Prior Art FIG. 1 shows an exemplar of a switch-based TDD RF LNA isolator, connecting between a base station transceiver (BST) port 10 and an antenna feed port 16. The feed to the BST port 10 is not shown. For example, BST port 10 may connect to the input (not shown) of a receiver unit (not shown) and to the output (no shown) of a transmitter unit (not shown). In the transmit mode the switches 20 and 24 isolate the RF amplifier 18 and connect the BST port 10 to the antenna port 16. In contrast, in the receive mode, the switches 20 and 24 isolate the BST port 12 from the antenna port 16, while providing a path for incoming RF communication signals from the antenna feed port 16 to the input 18A of the RF amplifier 18, and a path from the output 18B of the RF amplifier 18 to the BST port 10.
Another known apparatus and method for isolating signal output of the RF transmitter from the input of the RF receiving amplifier employs a particular arrangement of circulators. The operation of circulators is known to persons skilled in the related arts and, therefore, a detailed description is omitted here. Prior Art FIG. 2 illustrates an exemplar of a known circulator-based TDD RF LNA isolating apparatus. Referring to FIG. 2, the illustrated exemplar has a first circulator 12 fed at one port (12A) by the BST port 10, and a second circulator 14 having a port 14A receiving, through transmission path 15, output from port 12B of the first circulator 12. As seen by the relative position of ports 12B and 12A of the first circulator 12 along the circulator coupling direction 12R, signals entering port 12A couple to port 12B. Likewise, as shown by their relative position along the second circulator coupling direction 14R, signals entering port 14A couple to port 14B. Port 14B of the second circulator 14 connects to the antenna feed port 14.
Referring to prior art FIG. 2, it is readily seen that the circulators 12 and 14 form a directionally coupled transmit path for RF transmitter signals exiting the BST port 10 to reach the antenna feed port 16, and form a directionally coupled reception path for reception signals to propagate from the antenna feed port 14A to the input 18A of the RF LNA 18, and from the output 18B of the RF LNA 18 to the BST port 10. More specifically, with respect to the transmit path, a transmitter signal enters port 12A of the first circulator 12, leaves port 12B, propagates along path 15, enters port 14A of the second circulator 14, leaves port 14B and, finally, enters the antenna feed port 16.
With continuing reference to the prior art FIG. 2, as seen the selective coupling characteristic of the second circulator 14 substantially reduces transmitter signal entering port 14A from coupling or otherwise leaking to port 14C. This is significant, because such transmitter signal would otherwise enter the input 18A of the RF LNA 18 and, hence, be amplified along with desired reception signal.
The above prior art methods apparatuses for TDD have various shortcomings. A significant shortcoming that is common to both these depicted switch-based and circulator-based TDD RF LNA isolators is that no failure-mode bypass of the RF LNA is provided. Methods for providing such bypass have been proposed and described, but each has various shortcomings and/or lacks features, benefits and advantages provided by exemplary embodiments and aspects according to the present invention. One illustrative example is the apparatus and method recited and illustrated at U.S. Pat. No. 6,812,786, issued 2004 to Jackson, et al. (hereinafter referenced as “Jackson '786”). Jackson '786 controls a selector switch 192 and controls two PIN diodes connecting to one hybrid 156, to selectively switch from the RF amplifiers 152 and 154 to the isolator 190. The selector switch 192 and its control are required, and as well as the isolator 190.
For these and other reasons the present embodiments provide advancements in the art, having various described features, advantages, and benefits as well as others that will be apparent to persons of ordinary skill in the art upon reading this disclosure.